Solid-state light sources can incorporate, for example, one or more LEDs and optionally may include one or more phosphor materials. A typical conventional, solid-state light source is constructed from one or more packaged LEDs. Each LED package may contain one light emitting, multilayer semiconductor structure mounted on a substrate that includes appropriate electrical contacts. Alternatively, each package may contain one multi-layer semiconductor structure mounted on a substrate and include a wavelength conversion material consisting of phosphor particles that may be embedded in a transparent polymer. The wavelength conversion material usually covers the emitting area of the LED.
Both the LEDs and the phosphors used in conventional solid-state light sources have deficiencies that can be eliminated in order to provide less expensive light sources and to provide sources with higher optical outputs. In addition, the standard combined LED/phosphor package is bulky and is deficient in many ways. Some of the deficiencies of conventional solid-state light sources are described below.
Conventional LEDs are fabricated by epitaxially growing multiple layers of semiconductors on a growth substrate. Inorganic light-emitting diodes can be fabricated from GaN-based semiconductor materials containing gallium nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN) and aluminum indium gallium nitride (AlInGaN). Other appropriate materials for LEDs include, for example, aluminum gallium indium phosphide (AlGaInP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), diamond, boron nitride and zinc oxide (ZnO). Especially important LEDs for this invention are GaN-based LEDs that emit light in the ultraviolet, blue, cyan and green regions of the optical spectrum and AlGaInP-based LEDs that emit in the orange and red spectral regions.
The total thickness of the semiconductor layers for a conventional GaN-based LED is only about 3 microns. The layers are fabricated by epitaxially growing a layered semiconductor structure on a growth substrate using metal organic chemical vapor deposition (MOCVD), which has a very slow growth rate of approximately 0.1 micron per hour. This results in deposition times of tens of hours and makes the growth of thicker layers prohibitively expensive. The approximately 3-micron thick multilayer semiconductor structure is very fragile and will break easily if removed from the growth substrate to form a free-standing die. The semiconductor layers must therefore either remain attached to the growth substrate or, alternatively, be attached to a transfer substrate using wafer bonding techniques followed by removal of the growth substrate. The wafer bonding techniques are expensive and can be unreliable. The added steps increase the cost of manufacturing LEDs. Removal of the growth substrate can be done by a laser liftoff process, chemical processing or mechanical polishing.
The growth substrate for GaN-based LEDs is usually sapphire or silicon carbide and is chosen to closely match the crystallographic structure of the epitaxial layers. A transfer substrate, if utilized, can be a metal, another semiconductor material such as silicon or a ceramic material such as aluminum nitride. Such growth or transfer substrates may not suitable for the final LED device. For example, sapphire is a poor thermal conductor and is therefore not the most effective thermal conductor to direct heat away from the semiconductor layers. Thermal considerations are very important for LEDs, which generate a significant amount of heat during operation. The heat lowers the light output and operating lifetime of the LED. As LED sizes become larger, such heating effects become more important and can seriously degrade the light-output performance and lifetime of the LEDs.
In addition, the growth or transfer substrate may absorb some of the light emitted by the LED, thereby lowering the optical output. The substrate may also trap some of the light generated by the LED, resulting in an additional loss in optical output. Light trapping is caused by the high refractive index of the substrate relative to air and results in total internal reflection of emitted light back through the substrate and back through the epitaxial layers.
It would be desirable to develop thick, rugged LED chips that do not include either growth or transfer substrates and that can be easily handled without breaking. Different growth techniques will be required to make such a structure since MOCVD is too slow to fabricate thick multi-layer semiconductor structures.
In standard LED-based light source designs, the back side of the LED opposite the light emitting side is a reflective surface. It would also be desirable to develop LED chips that do not have a back reflecting surface and that can emit light from all sides. Eliminated the back reflecting surface can reduce the average optical pathlength of the emitted light within the LED structure, thereby reducing optical absorption within the LED and increasing the external quantum efficiency.
A conventional wavelength conversion material for solid-state lighting typically consists of a phosphor powder that may be embedded in a transparent polymer. The wavelength conversion material can be deposited, for example, as a dome that covers the output surface of the LED.
The standard approach to produce wavelength conversion materials begins by making bulk solid phosphors using solid-state processing as known in the art. These phosphors are then ground down to powders in the micron size range and deposited on a surface using a variety of deposition techniques such as settling, encapsulation within a polymer matrix or spray coating. Though relatively inexpensive, the phosphors generated using these methods suffer from high levels of dislocations and lattice defects. In addition, the compositional purity is also difficult to maintain. In the majority of cases, this does not represent a major problem because of the reduced excitation levels. It has been shown in accelerated aging studies, however, that very high excitation levels can degrade the output luminescence of powdered phosphors severely and impact overall life performance. These levels of high excitation exist within solid-state lighting applications. This is mainly due to the small size and concentrated flux density of the LED die itself.
Several material characteristics such as lattice defects, out-gassing, and compositional purity contribute to the problems of light output degradation and/or loss in efficiency for phosphor materials. It has been shown that polycrystalline and mono-crystalline phosphor films either grown on a substrate or as single crystal boules tend to exhibit much better luminosity and life characteristics than powders. In addition, every phosphor has a thermal quenching level that can degrade the output at the temperatures created by elevated excitation levels. In the case of powdered phosphors, this can be a major issue because the phosphor particles are usually isolated from any reasonable thermal conduction path. At very high excitation levels, the energy associated with less than unity quantum efficiency and Stokes shift losses can induce a significant localized thermal rise within the phosphor particles. The need exists for the creation of an improved thermal conduction path for the luminescent material. Also, the scattering created by the use of a powder can reduce the overall light output due to the backscattering and subsequent absorption of the generated light.
Mueller-Mach et al. in U.S. Pat. No. 6,696,703 disclose the deposition of a thin film phosphor directly on the LED die. However, as-deposited thin film phosphors have relatively poor wavelength conversion efficiency. A high-temperature annealing step is required in order to properly activate the phosphor. This annealing step can damage the semiconductor layers of the LED. In addition, the absorption cross-sections of most thin film phosphors are low, especially for blue and near ultraviolet (UV) excitations typically used for solid-state lighting. It is neither economical nor practical in most cases to create a sufficiently thick layer of luminescent material grown directly on the LED. Another drawback to depositing a phosphor directly on the LED die is that a large portion of the light generated within a deposited phosphor layer can be trapped due to total internal reflectance. The need therefore exists for a method to utilize high performance phosphors within an LED package such that the best phosphor can be used efficiently (e.g. with sufficient quantity, minimal backscatter, and maximum light extraction). The need also exists for a method to fabricate high efficiency phosphors without damaging the LED semiconductor layers.
Mueller-Mach et al. in U.S. Pat. No. 6,630,691 disclose a thin single-crystal phosphor substrate onto which an LED structure is fabricated by epitaxial growth techniques. However, single-crystal phosphor substrates are expensive and finding a single crystal phosphor substrate that has the proper lattice match to allow the growth of the LED structure can be difficult.
Ng et al. in US Patent Application No. 20050006659 disclose a planar sheet of a single-crystal phosphor that is placed over the output surface of an LED as a portion of a preformed transparent cap. However, single-crystal phosphor sheets must be grown by epitaxial processes or sliced from bulk single crystals of phosphor material. Single crystal phosphor sheets are therefore too expensive for most practical applications. Planar sheets of polycrystalline phosphors are not disclosed in US Patent Application No. 20050006659. Bonding the planar sheet of a single-crystal phosphor directly to the surface of the LED to improve heat dissipation in the phosphor sheet is also not disclosed.
A need exists to maximize the efficiency of wavelength conversion materials within a solid-state lighting application and to improve the thermal conductivity properties of the materials. In addition, a need exits for low-cost phosphors that have light extraction enhancements and the ability to control the level and type of scatter within the phosphor in order to enhance the overall conversion efficiency.
It would be desirable to replace the conventional wavelength conversion material with a solid wavelength conversion chip that could be bonded to the surface of an LED chip. This cannot be done with most types of LED devices since the wavelength conversion chip would cover up one or both of the LED electrodes and prevent attachment of electrical connections to the LED. It would be desirable, therefore, to incorporate an electrical interconnection means within the wavelength conversion chip.
A conventional LED package containing an LED and a wavelength-converting phosphor is bulky compared to the light emitting epitaxial layered structure itself. If many LED packages are used in the solid-state light source, the light source is significantly larger and thicker than necessary.
It would be desirable to form stacks of light emitting chips, where each stack includes one LED chip and one or more wavelength conversion chips. Such stacks of chips can be handled individually or combined with other stacks to form larger distributed light sources. Such stacks of light emitting chips could be utilized for applications such as backlights for liquid crystal displays (LCDs) or for general lighting applications such as room lighting. It would also be desirable if the LED chip used in such a stack has a thick, rugged multilayer semiconductor structure and that the chip does not retain the growth substrate nor utilize a transfer substrate. Eliminating the transfer and growth substrates can improve the thermal conduction properties of the light emitting chips.
It would also be desirable to fabricate a solid-state light source that is a stack of two or more LED chips and optionally includes one or more wavelength conversion chips. Such stacks could be handled and used individually or formed into arrays to construct a distributed light source.
It would also be desirable if the stacks of LED chips and wavelength conversion chips could be electrically connected in series, parallel or anti-parallel configurations or in some combination of series, parallel or anti-parallel configurations. Anti-parallel electrical configurations are desirable if the electrical power source is an alternating current source.
U.S. Patent Publication No. 20050269582 discloses a ceramic phosphor layer bonded to a conventional LED. In one example, the LED is a flip chip device with both electrodes on the side of the LED opposite the ceramic phosphor so that no electrodes are in the way when bonding the ceramic phosphor layer to the LED. The LED includes a growth substrate that is still attached to the top surface of the semiconductor layers of the LED. The phosphor layer is also bonded to the growth substrate, but on the side opposite the semiconductor layers. In a second example, the p-contact layer of the LED is attached to a transfer (host) substrate and the ceramic phosphor layer is attached to the n-layer opposite the transfer (host) layer. The n-contact is adjacent to the ceramic phosphor layer and on the same side of the LED as the phosphor layer. The original growth substrate has been removed but the transfer substrate remains with the device.
U.S. Patent Publication No. 20050269582 does not disclose LEDs that have neither a growth substrate nor a transfer substrate as an element of the LED die. U.S. Patent Publication No. 20050269582 does not disclose LEDs where the n-type layer, the p-type layer or both the n-type layer and the p-type layers are thick enough so that the multilayer semiconductor structure of the LED is rugged and may be handled as a free-standing chip without having the growth or transfer substrate still attached. In addition, U.S. Patent Publication No. 20050269582 does not disclose wavelength conversion chips that include an electrical interconnection means. U.S. Patent Publication No. 20050269582 also does not disclose a stack of light emitting chips where one chip is an LED chip and another chip is a wavelength conversion chip that includes an electrical interconnection means. U.S. Patent Publication No. 20050269582 does not disclose a stack of light emitting chips where at least two of the chips are LED chips and where the stack optionally includes at least one wavelength conversion chip.
Many types of conventional solid-state light sources that emit high output lumens or high output power attempt to generate the required lumens or power from one large LED die or from an array of closely spaced LED die. Although useful for light sources requiring a small emitting area or etendue, this type of light source has two deficiencies for general lighting applications such as room lighting. One deficiency is that the high output intensity from such a concentrated source can exceed eye safety standards and can be a safety hazard. It would be desirable instead to make large-area, distributed light sources using many smaller light source chips so that light intensity safety standards are not exceeded. To accomplish this, the light source chips need to be inexpensive and easy to handle. It would also be desirable if the light source chips were constructed as stacks of LED chips and wavelength conversion chips.
For general lighting applications, a light source brightness of less than 1000 ftL (foot-lamberts) is preferred. Assuming such a brightness level on a lambertian-emitting surface would indicate that the area of a 1000 lumen source would be around 1 square foot. Presently only 10 square millimeters of LED emitter area are required to generate 1000 lumens at a current density of 1 ampere per square millimeter and with a total drive power of less than 20 watts. However, the same 1000 lumens can be generated for less than half the input power if the LED die are operated at their maximum efficiency point. The die area required to do this is larger but still represents less than 0.1% of the 1 square foot area needed to make the 1000 lumen distributed light source.
A conventional high powered LED typically operates at high current density, for example 1-2 amperes per square millimeter of LED area, resulting in lower external quantum efficiency than would be the case if the LED were operating at lower currents. Usually the highest external quantum efficiency for the LED device is obtained at a much lower current density of about 0.1-0.2 amperes per square millimeter of LED area. Using a single LED die operating at high current density instead of several smaller LEDs operating a low current density is usually dictated by the high cost of packaging multiple LED die. It would be desirable to develop lower cost methods to manufacture LEDs so that one large LED can be replaced by several smaller light source chips operating at the most efficient current density. This would allow the low cost production of a 1000 lumen sources with 1 square foot area, for example.
An additional problem with high powered light sources is that a single, high-powered LED die or a closely spaced array of LED die can produce a significant amount of heat that must be dissipated quickly to prevent the die from overheating. Metal heat sinks with large area fins are generally required for convection cooling of such devices. Unlike a 1000 lumen point source, a more desirable 1000 lumen distributed source that covers a 1 square foot area would not need any thermal management due to the large surface area available for cooling. A naturally convection-cooled surface can easily dissipate 30 to 50 watts with a reasonably small increase in temperature. It would be desirable to combine this cooling mechanism with the improved efficiency of operating a distributed array of small LED light source chips at low current densities in order to create an efficient, low cost, uniform light source useful for a variety of applications.
Conventional LED-based light sources are cooled by a heat sink in thermal contact with the LED die. The LED die includes either a growth substrate or a transfer substrate. Heat flows from the LED semiconductor layers, through the growth or transfer substrate and through the heat sink to ambient. The heat sink may include fins or other types of structures to transfer heat to an ambient fluid, such as air or water. It would be desirable to develop LED-based light sources where the LED die do include the growth substrate or a transfer substrate and where the LED die can be cooled without a special heat sink. In such cases, heat will flow directly from the light source to an ambient fluid such as air or water.
The deficiencies of conventional solid-state light sources described above can be eliminated by the various embodiments of this invention that are described below in the summary, the figures and the detailed descriptions of the preferred embodiments.